The alignment of an optical instrument when it operates at two different wave-lengths is a difficult undertaking, especially when one of the wavelengths is outside the visible photopic electromagnetic spectral region. Due to the difference of the index of refraction at the two wavelengths, the rays are bent at different rates when they are refracted through a lens. The measurement becomes particularly difficult because aligning the optical instrument where an infrared wavelength is employed requires special instruments. One may measure the effect the rays will have on appropriate detectors under controlled conditions, or simulate laboratory conditions using special instruments. However, these indirect measurements often give uncertain results and become the cause of errors.
Thus, what is needed is a method of alignment verification that allows the operator to use his/her eyes to align an optical instrument.